[ TECHNOLOGY ]
A modified version of thawing in air is possible with vacuum equipment that operates at low temperatures and with high humidity. When the air is evacuated from the airtight unit the moisture condenses, is deposited on the frozen fish product and releases a significant amount of heat which speeds up the thawing process. This method is particularly effective because the condensing water vapour penetrates into even the tiniest gaps and cracks in the frozen fish blocks. These systems are often equipped with sterilization units in order to prevent the development of bacterial organisms. Modern vacuum thawing systems are not only gentle to the products because they transfer the heat evenly onto them and keep them constantly moist, but also very hygienic. An alternative to thawing in the air is to use water. This can be done by direct immersion, by spraying water onto the frozen fish, or by a combination of the two methods. Water as a medium for transferring heat energy during thawing has several advantages. It is a denser medium than air; it can be heated well, and the thermal energy is stored longer. From an energy point of view it thus makes more sense to run the water in a circuit and use it a number of times over. The water has close contact with the product that is to be thawed because it wraps itself around the fish closely www.eurofishmagazine.com
3X Technology
Sensors monitor the thawing process, especially the temperature curve and humidity. If necessary, the ambient air is cooled. Air blast thawing systems allow “just in time” thawing of frozen products so that they are available at exactly the time they are required. This method is suitable for both batch processes and continuous thawing within processing lines.
Products are often thawed by immersing them in a water bath. The water has the advantage of keeping the product surface moist, but this method is best used for whole fish as fillets left lying in water can lose flavour.
and transfers heat effectively. However, these advantages are counterbalanced by some disadvantages. Fish fillets that are left lying in water for a longer period are washed out and lose flavour and so this thawing method is more suitable for whole fish. It is also problematic that microbes accumulate in the heated water, and these can contaminate all the fish during thawing. This is especially true for systems where the water is used several times. At its simplest, defrosting in water can be carried out in a tank with a continuous flow of water. Automated thawing systems have their own sensors and control units to enable continuous monitoring of all important process parameters and thus their optimal adjustment. Thawing systems in which frozen blocks are thawed by spraying with water are often constructed so that
the outer layers of fish fall off the block onto conveyor belts so that they can be removed immediately. Another advantage is that the fish remain moist and the skin surfaces which are often germ-laden are constantly rinsed. Some thawing systems combine the immersion and spray methods with each other. Here the frozen fish are usually in baskets which circulate alternately through an immersion bath and a kind of shower facility in which water is sprayed from above onto the baskets. This is very effective because the heat is transferred directly and distributed uniformly over the entire surface of the product that is to be thawed.
Modern technologies shorten thawing times Electric thawing methods are even faster and more efficient. They have hardly been able to
assert themselves in industrial practice yet, however, because the technology required is relatively expensive, both to purchase and to operate. An extremely effective way to thaw frozen fish products, and probably the fastest of all, is based on the electrical resistance the frozen blocks display to flowing current. In “dielectric heating” the frozen fish blocks are literally jammed between two parallel metal plates between which current flows. Frozen fish is a poor conductor of electricity but the electric resistance decreases rapidly once the fish is warmer. The basic principle of this thawing method resembles an electric heater in which the filament heats up when electricity passes through it. Due to its high thawing capacity dielectric heating is especially suitable for large quantities of fish or applications that need to respond flexibly to changing
Eurofish Magazine 4 / 2013
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